As lithography process technology moves to smaller and more closely spaced features, it becomes more difficult to print the original design onto a wafer due to resolution limits. A primary limitation is how closely features can be spaced due to optical limitations. This is so even if resolution enhancement techniques (RET) such as optical proximity correction (OPC), immersion scanning, and phase shift masks are employed.
Double patterning is a technique that has been considered to achieve more closely spaced features. In double patterning, two reticles are used to print split portions of an original pattern onto a hard mask that, in turn, is used to transfer the original pattern to the substrate. To print two patterns on the hard mask, the original pattern is split into two or more patterns where each has a narrowest pitch that is wider than the narrowest pitch in the original pattern. Each of the split patterns is printed to a separate reticle. First one reticle is exposed onto a photoresist above the hard mask. After transferring the first pattern from the photomask to the hard mask, the photomask is removed, and another photomask is added above the hard mask. Next, the second reticle is exposed onto the second photoresist. After transferring the second pattern to the hard mask, the second photomask is removed. Finally, the combined pattern is transferred from the hard mask to the wafer.
However, for such a technique to be effective the exposure of the two split patterns needs to be precisely aligned such that the original pattern is achieved. However, lithography techniques have limitations in how precisely the two patterns can be aligned.
Thus, improved techniques for forming devices, such as semiconductors, having narrow pitches are desired.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.